In optical communication engineering, coherent optical heterodyne and homodyne detection techniques are currently the subject of intensive research and development because of the extremely high transmission capacity of such systems, cf. for example, M. Rocks, "Optischer Uberlagungsempfang: Die Technik der Ubernachsten Generation glasfasergebundener optischer Nachrichtensysteme", Der Fernmelde-Ingenieur, No. 2, 1985, pp. 1-38. The methods described there are commonly used in broadcasting systems employing optical carrier frequencies in the 200-THz range. As shown particularly in FIG. 3 of that article, in coherent optical heterodyne and homodyne detection systems, the optical signal carrying the useful information is combined with the output beam of a continuous-wave local-oscillator laser. In the heterodyne detection technique , this results in an electric intermediate-frequency signal from which the useful information can be recovered with high sensitivity by electrical demodulation. A similar arrangement is described in W. A. Stallard et al, "Electro-optic frequency translators and their applications in coherent optical fibre systems", Br.Telecom. Technol. J., Vol. 4, No. 4, Oct. 1986, pp. 16-22 (especially FIG. 4). In many broadband communication applications, e.g., at the subscriber level of a broadband ISDN, bidirectional optical transmission is desired. In a coherent optical transmission system, this would necessitate equipping each transceiver at the subscriber end with two stabilized, narrow-band single-mode lasers, namely with a narrow-band transmit laser capable of being modulated and a narrow-band, frequency-controlled localoscillator laser. In other words, if conventional techniques were applied to coherent optical transmission, a transmitter/receiver unit would require a transmitter oscillator laser and a local oscillator laser for heterodyne or homodyne detection. To be able to separately transmit and receive on each channel in a bidirectional coherent optical single- or multi-channel system, both lasers require narrow linewidths, which lie between 0.1 and 50 MHz depending on the detection technique used, complex frequency control loops, e.g., for intermediate frequency control during heterodyne detection, a recognition system for specific optical reference channels, and optical isolators before the output to prevent optical feedback. Both single-mode lasers must thus be narrow-band, frequency-stabilized, and tunable. The transmit laser must also be capable of being RF modulated and must have an external modulator which should operate at bit rates above 1 Gb/s.
Thus, a coherent optical transmitter/receiver unit containing two frequency-stabilized, narrow-band single-mode lasers with the associated control systems and components is very complex and costly.